Polypropylene as a Turntable Base Material

Hi Ait,

This topic was one of the most contentious ones during the late Winter and early Spring of 2000 - when the original Teres project was its most active. In December, 1999 through January, 2000, the original Teres-5 worked through the basic parts design, after which point we frozen the design and invited the "public" into the project. It was at this point that peoples' attention turned to the design of their bases - having some 5 months' time before the delivery of their parts.

There was a split into two main camps, which someone (I think it was Ken Schei) designated as the "stiffies" and the "woodies". Of course, those of our friends from the British Isles would contend that a woodie is a stiffie, but I digress ...

The argument espoused by the "stiffies" was that although the frequency of a rigid piece of metal is quite high, and centered in the most sensitive area of human hearing, that this problem is negated by our good ol' friend the RIAA curve which attenuates high frequencies. They further argued that these higher frequencies are lower in amplitude and are easier to damp. Their approach in general centered on working from a perspective of damping a ringing substance and letting the RIAA equalization deal with the rest.

The "woodies" favored damping over rigidity, and for the most part they didn't work much at stiffening their bases. Their philosophy centered on using a material which could not easily be excited. The argument against soft, lossy bases has centered around its poor dimensional stability (both thermal and mechanical), combined with the additional challenge of draining vibrations away from the bearing due to the dissimilar materials interface (e.g. brass bearing --> wood base). The more similar the material interface is, the more vibration is transmitted and the less it is reflected.

We all know the different directions taken by both the DIY-ers as well as by Teres / Galibier / Redpoint, and it doesn't bear repeating here.

Because yours is a DIY project, I would advocate a flexible architecture which would allow you to experiment - adding a thin sheet of aluminum (ca 1/8" to 1/4" thick) to the top surface. If you go modular, you can experiment to your heart's content. You may not be in a position to generalize your knowledge and your discoveries beyond the context of your own turntable, but the good news is that it will be your design and will suit both your listening tastes as well as your system.

Cheers,
Thom @ Galibier

Thom: That's an interesting point about the RIAA curve. When I mentioned it above, I was thinking from the standpoint of the curve as it's recorded on the record, and how that would impact excitation of the machine through the stylus/groove interface, instead of from the standpoint of how it's applied in the phonoamp. I guess those two factors would tend to offset each other to some degree in isolation, but factor in the re-equalized sound as produced by the speakers and introduced back into the loop through the air and the gear rack, and avoiding a lower-frequency resonance would seem to make more sense...

Thom, first of all, thanks. That was one of the most useful responses I've ever gotten on a discussion board.
I don't know why this didn't strike me earlier, but a few years ago I was involved in some research with a major US National Lab centering around using both active and passive acoustic spectroscopy to determine changes in the texture of materials being heated inside a sealed steel pressure vessel. One of our biggest challenges was acoustic impedance matching, since that determined the degree of penetration of the sound waves (and we were exploring everything from audible to ultrasound) through the vessel, into the materials of interest, back through the vessel, and to our sensor unit. There is actually quite a body of research in this area - I'm going to go back through my notes and references now, since the light that just went on in my head is telling me that proper impedance matching is the key to properly draining vibrations from where they shouldn't be.
The sheet of aluminum you suggested is one way of coupling the dissimilar materials - I'm thinking that the use of specific acoustical coupling materials (think about the gels the techs use when giving you an ultrasound exam to couple the metal probe to your skin) may produce a much better effect.
Thanks for the inspiration - this should be fun!

For those who would like to explore acoustic impedance matching in the design of their systems, here are some very useful tables of acoustic properties (Z is the acoustic impedance). Remember, the farther apart the impedances, the more acoustical energy is reflected instead of transmitted at the interface. Have fun!

http://www.ondacorp.com/tecref_acoustictable.html

This is great stuff! Thanks for the link Ait.

As you reflect on all of this, bear in mind that there are quite a few different ways to solve a problem. This will help you keep your sanity and enjoy the ride.

If you think about Frank Schröder's tonearm (a "woodie" if there every was one), he has to approach the problem entirely differently. To my understanding (never discussed this with Frank) his problem involves draining all of the energy before (or at least by the time) it reaches the end of the arm. There's not much of an energy drain through the bearing and I don't expect he'd want the vibes making a return trip to the headshell/cartridge.

Of course, from his point of view, this provides a degree of isolation from vibes traveling through the stand into the base and then into the tonearm.

Certainly, I love Frank's tonearms, but I approach a turntable design more from a coupling perspective - trying to eliminate as many "wiggles" as possible. One advantage (to my way of thinking) of a rigid base is that it better fixes the relationship between the tonearm and the bearing. We've already mentioned acoustic impedance matching, so I won't get into that again. This fixed relationship would seem to me to afford better speed stability at the micro level - which is perceived as an absence of FM distortion (way below the perception threshold of wow and flutter).

Of course, nothing comes for free in any physical design which is what makes this game so interesting ... watching solutions come literally out of left field.

Cheers,
Thom @ Galibier